SHOULD, OUGHT AND MUST

Should and ought can often replace each other since they are very similar.

For Example:
1. They ought to be more practical, shouldn't they?

Should and ought are also used to talk about obligation and duty, to give advice, and to say what we think is right for people to do or have done. Using should is much more frequent than ought.

Some Examples:
1. You should/ought to go watch the new film coming in the cinema, it’s very controversial.
2. You should/ought to watch him dance!

Should and ought are not used in polite requests.

For Example:
1. Could you move your chair a little? (You do not say – You should/ought to move your chair a little.)

Should and ought are also both used to talk about logical probability.

Some examples:
1. I've bought 2 dozens of donuts – that should/ought to be enough.
2. That should/ought to be Hannah opening the door.

Unlike should, ought is followed by a to-infinitive.

Should and ought is similar to the meaning of must, but is just stronger or more definite. It expresses great confidence that something will happen, or that something is true. Should and ought express less confidence.

Please see comparison below:
a. The doctor said I must take eight hours of sleep every day. (It means – It is an order that must be obeyed.)
b. You really ought to take eight hours of sleep every day. (I t means – You should follow this advice.)
c. Hannah must be at school by now. (It means – It is sure that she is at school.)

To make instructions sound more polite, should can be used instead of must.